Disclaimer The lecture slides for this semester are based upon the - - PDF document

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01 Introduction to Language Paradigms CS631 Fall 2000 1

Programming Paradigms

Primary Language Classification

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Disclaimer

The lecture slides for this semester are based upon the slides used by Dr. Adam Webber. Credit and thanks are due to Dr. Webber; mistakes are mine.

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Related Reading

Chapter 1 Programming Languages Concepts and Constructs, Ravi Sethi

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Overview

“Don’t fight the paradigm” “Use the right tool for the job”

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Why Study Programming Languages?

languages, the easier it will be to learn the “next great language.”

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Classification: Four Language Families

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The Imperative Language Paradigm

statements.

“environment” (memory, devices, etc.).

be done in the order specified.

computer hardware

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Imperative Example: Factorial in C

Imperative characteristics: assignment statements (= and *=) loop depends on side-effects sequence of statements with implied ordering void main(int argc, char *argv[]) { int n = atoi(argv[1]) ; int sofar = 1; while (n > 1) sofar *= n--; printf (“%d\n”, sofar); }

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Imperative Languages

– COBOL – BASIC – C – Fortran – Ada – Pascal – and many others

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Imperative Paradigm Strengths

most easily.

hardware works, it is easy to translate into efficient machine language.

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Imperative Language Drawback #1: Difficult to Reason About

themselves that the program works.

to find a way to formally prove that a program is correct.

can’t reason about any part of the program independently of the other parts. . .

a = f(x,y); b = f(x,y); // Does a equal b? Or f might change a: int f(int x, int y) { a = 0; return x + y; } f might not be referentially transparent: int f(int x, int y) { return system_clock; } Or a and x might be aliased: int a; int& x = a; ... a = f(x,y); b = f(x,y);

Example

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speed up your program if you can make use of more than one processor.

do not depend on being executed in the standard

• rder.

imperative languages make this difficult to do automatically.

Imperative Lang. Drawback #2 (1): Hard to Parallelize

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Imperative Lang. Drawback #2 (2): Parallelization, Example 1

a = 0; b = a; b = a; a = 0; For each group of C statements below, is it possible for a non-standard order of execution to give different results from the standard order? a = 0; a = 1; a = 0; b = 0;

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*p = 1; *q = 2; *p = 0; *q = 0; For each group of C statements below, is it possible for a non-standard order of execution to give different results from the standard order?

Imperative Lang. Drawback #2 (3): Parallelization, Example 2

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for (int i = 0; i < 100; i++) a[i] = i * 2; for (int i = 1; i < 100; i++) a[i] = a[i-1] * 2; For each group of C statements below, is it possible for a non-standard order of execution to give different results from the standard order?

Imperative Lang. Drawback #2 (4): Parallelization, Example 3

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The Object-Oriented Paradigm

strictly-enforced interface to be used by other

• bjects.

• f such objects.

class MyInt

{

private int value;

public MyInt(int value) { this.value = value; } public int getValue() { return value; }

public MyInt getFactorial()

{

return new MyInt(fact(value));

}

private int fact(int n)

{

int sofar = 1; while (n > 1) sofar *= n--; return sofar;

} }

“I am a MyInt object”

OO Example Factorial in Java

“I hold an int value” “I can be created” “I can tell you my value” “I can return a MyInt object

for my factorial... ”

“…but how I calculate my

factorial is my own business ”

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calls (side effects).

imperative code

The Object-Oriented Paradigm Is it really just imperative?

Object-oriented programming is really a variation

• n the theme of imperative programming.

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Object-oriented Languages

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Object Oriented Benefits (1)

Better Organization of Large Programs

details from other objects

from other objects (makes code reuse simpler) Encapsulation and inheritance also help moderate the drawbacks of imperative programs...

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– OOP helps since private properties of objects can be

reasoned about in isolation from the rest of the program.

– OOP helps since the strict interfaces among objects

make good places at which to divide up the computation among machines.

Object Oriented Benefits (2)

Mitigate Drawbacks of Imperative

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OO Drawbacks

imperative programs, but at a cost:

– Programs can be longer – Languages can be more complex – Paradigm can take longer to master

Bad OOP code is often worse than bad imperative code

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The Functional Paradigm

definitions.

detectable permanent effect on memory, devices, etc.

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Functional Example Factorial in ML

fun fact x = if x = 0 then 1 else x * fact(x-1); Functional characteristics: No assignment statements Recursion (not necessarily present) Referential transparency

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Functional Languages

– Common Lisp, Scheme, T

– Haskell, Gofer, ML

– FP

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Functional Benefits (1) Easier to Parallelize

We know these can be evaluated simultaneously, without even seeing the definitions of g and h. f( g(a), h(a) )

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Functional Benefits (2) Easier to Reason About

transparency we know that calling the same function with the same parameters always produces the same result.

( ) ( ) ( )

2 2 1 1 2 1 2 1

, , y x f y x f y y x x = ⇒ = ∧ =

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Functional Paradigm Drawbacks

curse

– Pro: You can reason about the program – Con: I/O is technically not allowed

– Recursion can be a very difficult concept – No assignment statements

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The Logic Paradigm

assertions and rules of inference.

“goal”) from the part already given.

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Factorial Written in Prolog

fact(0,1). fact(X,Y) :- X>0, R is X-1, fact(R, Z), Y is X*Z. This is a logical definition of the relation, “Y is the factorial of X”. ?- fact(4,10). No You can then ask, “can you prove that 10 is the factorial of 4?” (No!) ?- fact(4,X). X = 24 Or you can ask, “can you prove that X is the factorial of 4?” (Yes, if X is 24!)

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Logic Programming Languages

queries)

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Logic Programming Benefits

certain kinds of problems:

– dynamic database of facts – pattern-matching, backtracking search

– You express the problem logically, without

saying exactly how to solve it

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Logic Programming Drawbacks

– Side effects such as I/O require forcing imperative

paradigms onto logic paradigm

– Is the program efficient? – Will a program terminate?

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Complexity Results

languages have equivalent power.

mathematical models of computation: Turing machines, lambda calculus, etc.

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Which Paradigm is Best?

paradigms than others.

language may be based on non-paradigm issues. But match the paradigm to the language--don’t fight the paradigm. Forcing the wrong paradigm on the language results in bad code.

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Fighting the Paradigm: Imperative ML

fun fact n = let val i = ref 1; val xn = ref n in while !xn>1 do ( i := !i * !xn; xn := !xn - 1 ); !i end; ML makes it hard to use assignment and side-effects. But it is still possible: Don’t write your ML code this way!

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Fighting the Paradigm: Imperative Java

Java, more than C++, tries to encourage you to adopt an

• bject-oriented mode. But you can still put your whole

program into static methods of a single class: class Fubar { public static void main (String[] args) { // Put the whole program here and/or // use nothing but static methods } }

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Fighting the Paradigm: Functional Pascal

procedure ForLoop(Low,High: Integer): Boolean; begin if Low <= High then begin {for-loop body here} ForLoop(Low+1, High) end end;

Any imperative language that supports recursion can be used as a functional language:

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Conclusions

weaknesses

depends on the language in which you are trying to express it. Don’t fight the paradigm--use the right paradigm for the language.

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Review Questions

paradigms?

paradigm?